Pressure swing adsorption gas separation method and apparatus

ABSTRACT

The present invention is a gas separator for separating a gas mixture into a product gas. The gas separator has an adsorbent bed including a separation chamber with first and second ports and a molecular sieve material contained in the separation chamber. A first pumping chamber is connected to the first port. A first valve regulates a flow of the gas mixture between the first port and the first pumping chamber. A first piston is located in the first pumping chamber. A second pumping chamber is connected to the second port. A second valve regulates a flow of the product gas between the second port and the second pumping chamber. A second piston is located in the second pumping chamber. A drive system coordinates operation of the first and second pistons and the first and second valves in a cycle including a pressurization stage, a gas shift stage, and a depressurization stage.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority from U.S. patent applicationSer. No. 10/044,791 filed Jan. 10, 2002, for “PRESSURE SWING ADSORPTIONGAS SEPARATION METHOD AND APPARATUS” by Theodore W. Jagger, Alexander E.Van Brunt and Nicholas P. Van Brunt.

[0002] This application also claims priority from ProvisionalApplication No. 60/261,630 filed Jan. 12, 2001, for “PRESSURE SWINGADSORPTION GAS SEPARATION METHOD AND APPARATUS” by Theodore W. Jagger,Alexander E. Van Brunt and Nicholas P. Van Brunt.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to the separation of a selected gasfrom a mixture of gases by pressure swing adsorption. The primary goalis to maximize the ratio of selected gas volume to energy input whileminimizing the ratio of mechanical volume of the separator to theselected gas volume. The present invention separates, from the gasmixture, a concentration of one or more components of a gas mixture fordelivery or storage.

[0004] Pressure swing adsorption (PSA) is a frequently used method toseparate one component of a gas mixture. For example, pressure swingadsorption is used to separate concentrated oxygen from air and thendeliver it to a patient for medical purposes. A common use in homemedical care is the delivery of 90-95% concentrated oxygen, derived fromthe atmosphere, at rates up to 6 liters per minute for the treatment ofemphysema or other diseases of the lungs in the home. The machines usedfor this purpose are large, bulky, heavy, and require a large amount ofpower to operate, thus making battery power impractical. Patients musthave a supply of bottled oxygen available when they leave their homebecause use of an oxygen concentrator outside the home is not convenientor practical. Use of bottled oxygen is undesirable because of itsdisadvantages: limited operating capacity, heavy weight andhazardousness.

[0005] PSA is widely used in industrial gas separation processes aswell. Industrial PSA processes vary in the type of gas mixture used andselected gas separated.

[0006] Generally PSA involves injecting a mixture of gas into a gasseparation chamber having an adsorbent bed or molecular sieve bed. Onegas is readily adsorbed when pressurized above atmospheric pressure inthe adsorbent bed, while the other gas is less adsorbed. Both of theseparated gasses may be utilized or one may preferentially be used whilethe other is vented as waste. The adsorbed gas in the adsorbent bed isreleased upon lowering the gas separation chamber to the originalatmospheric pressure, at such time purging the adsorbent bed. In orderto achieve sufficient concentration of the separated gas, two or moreadsorbent beds are used in either sequential or multi-processing modes.It is common to purge the bed of the adsorbed gas by using a portion ofthe product gas in order to improve the efficiency of the process.

[0007] The selected gas in the mixture can be either adsorbed with theremaining mixture, vented to atmosphere, or otherwise removed.Alternatively, the undesirable components may be adsorbed leaving theselected gas to be passed on for storage or immediate use. The adsorbedcomponent is then discharged as a waste gas, stored or utilizedimmediately dependant upon the application.

[0008] To further clarify, the following is a description of a typicalcontinuous process.

[0009] (1) Feed gas mixture (A+B) into a container with an adsorbentbed, at some pressure above atmosphere until the bed is saturated.

[0010] (2) Gas B must be adsorbed and stopped before it exits theproduct end. Gas A is moved to temporary or permanent storage from theproduct end.

[0011] (3) Reduce pressure on adsorbent bed.

[0012] (4) Extract gas B from the feed end while taking a fraction ofgas A and feeding it back into the product end to purge gas B.

[0013] (5) Stop feeding gas A into the product end before it exits fromthe feed end.

[0014] (6) Return to step 1.

[0015] An example of this process is disclosed in U.S. Pat. No.5,415,683 entitled “VACUUM PRESSURE SWING ADSORPTION PROCESS”.

[0016] Methods of providing portability, as disclosed in U.S. Pat. No.4,971,609 entitled “PORTABLE OXYGEN CONCENTRATOR” and U.S. Pat. No.4,826,510 entitled “PORTABLE LOW PROFILE DC OXYGEN CONCENTRATOR”,attempt to reduce the physical packaging design and provide on-demandflow, thereby improving efficiency and portability. These designs areseverely limited because they do not improve the inherent low efficiencyof the PSA process. The prior art requires an amount of energyimpractical for sustained battery operation, a small compact size andlightweight apparatus, while still producing the necessary flow rate andproduct gas concentration. Inefficiencies arise in the PSA process fromthe following sources: (a) resistance to gas flow through the adsorbentbed, (b) energy losses in the pressurization/depressurization process,(c) irreversible thermal losses, and (d) inefficiencies in compressors,gas pumps and valves. Negating these inefficiencies while maintainingthe desired flow rate and concentration is required in order to achievea smaller, lightweight overall machine package capable of batteryoperation.

BRIEF SUMMARY OF THE INVENTION

[0017] The present invention is a gas separator device using pressureswing adsorption to separate from a gas mixture the concentration of oneor more components of that mixture.

[0018] The present invention is a gas separator for separating a gasmixture into a product gas. The gas separator has an adsorbent bedincluding a separation chamber with first and second ports and amolecular sieve material contained in the separation chamber. A firstpumping chamber is connected to the first port. A first valve regulatesa flow of the gas mixture between the first port and the first pumpingchamber. A first piston is located in the first pumping chamber. Asecond pumping chamber is connected to the second port. A second valveregulates a flow of the product gas between the second port and thesecond pumping chamber. A second piston is located in the second pumpingchamber. A drive system coordinates operation of the first and secondpistons and the first and second valves in a cycle including apressurization stage, a gas shift stage, and a depressurization stage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a diagram of a pressure swing gas separator of thepresent invention.

[0020]FIG. 2 is a graph demonstrating the relative timing of pistons,valves and gas separation of the present invention, during a pressureswing cycle.

[0021]FIG. 3 is a table summarizing the functional stages of the gasseparator of the present invention.

[0022]FIG. 4 is a graph demonstrating the relative timing of pistondisplacement, valve opening and closure, and pressure relative to eachpiston during the pressure swing cycle.

[0023]FIG. 5 is a diagram of product gas and gas mixture in an adsorbentbed during the pressure swing cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024]FIG. 1 is a diagram of pressure swing gas separator 10 of thepresent invention for separating from a gas mixture the concentration ofone or more components of that mixture (product gas). Separator 10includes adsorbent bed 12, first pump 14, second pump 16, first valve18, second valve 20, controller/energy source 22, a gas mixture inlet24, first outlet 26 and second outlet 28. A gas mixture enters gasseparator 10 through inlet 24. Waste gas, the gas mixture minus theproduct gas, exits gas separator 10 at first outlet 26 (feed end). Theproduct gas exits gas separator 10 from second outlet 28 (product end).

[0025] Adsorbent bed 12 includes separation chamber 30, which has firstport 32 and second port 34. A molecular sieve material 36 is containedwithin separation chamber 30, such as an adsorbent pressure swingpreferential material (e.g. zeolite).

[0026] First pump 14 includes pumping chamber 40, first piston 42,piston rod 44, crank shaft 46, and first drive motor 48. Pumping chamber40 is connected to first inlet 24, first outlet 26 and to first port 32of adsorbent bed 12.

[0027] Second pump 16 includes second pumping chamber 50, second piston52, second piston rod 54, second crank shaft 56 and second drive motor58. Second pumping chamber 50 is connected to second outlet 28 and tosecond port 34 of adsorbent bed 12.

[0028] First valve 18 controls gas flow between first pumping chamber 40and first port 32 of adsorbent bed 12. Second valve 20 controls gas flowbetween second pumping chamber 50 and second port 34 of adsorbent bed12. Controller/energy source 22 operates as a drive system to controloperation of first and second drive motors 48 and 58 (thereby first andsecond pistons 42 and 52) and first and second valves 18 and 20.

[0029] The preferred embodiment of the gas separator 10 requires acomplex timing sequence of valve opening and closure, piston movementrelative to one another and the valves, and the sizing of the pistons(volumetric displacement). All these interactive variables are requiredfor each product gas.

[0030]FIG. 2 is a graph demonstrating the relative timing of pistons,valves and gas separation in the present invention during a pressureswing cycle (and includes a portion of the structure shown in FIG. 1).Axis 60 of the graph in FIG. 2 shows degrees/unit of time and axis 62shows piston displacement. Adsorbent bed 12, including gas mixture 62and product gas 66 ratios during the pressure swing cycle, is shownlongitudinally along the middle of the graph. The heavy lined sinusoidalwaveforms 68 and 70 show the path of piston displacement for first andsecond pistons 42 and 52, respectively. The actual optimized waveformwould not necessarily be an exact sinusoid, but for clarity this is usedin the drawing. Lines 72 and 74 are used to show the opening and closingof first and second valves 18 and 20, respectively. The dark horizontalsolid lines are used to indicate the closing of the valves while thedashed lines indicate the opening of the valves.

[0031] Absorbent bed 12 is shown elongated along the graph to illustratewhat happens to the gas mixture as pistons 42 and 52 go through theirdisplacement cycle. Note in particular the concentration of the productgas in the bed as it relates to the timing of the piston displacementcycle. In this illustration the concentration of the product gas in thebed increases during the pressurization stage, from 0 degrees to about110 degrees. At 110 degrees, the gas shift stage is underway with firstvalve 18 open and first piston 42 moving upward. The pressure in theadsorbent bed is pushing second piston 52 upward during this timehelping to pull the product gas out of the adsorbent bed and recover thepotential energy of the compressed gas. When second valve 20 closes, thedepressurization stage and waste gas removal begins. First piston 42moves downward depressurizing the bed, thereby removing the waste gasfrom the bed and preparing for the entry of a new gas mixture. Toseparate different gasses at the required volumetric flow requires newparameters to be selected for the volumetric displacement, valve timingand piston cycles relative to one another.

[0032]FIG. 3 is a table summarizing the functional stages of the gasseparator. The stages include a pressurization stage, a gas shift stageand a depressurization stage. A significant difference in this inventionfrom prior art gas separators is the minimization of energy loss atevery stage in order to maximize the ratio of product gas volumeproduced to energy used.

[0033] As seen in FIG. 2 and described by FIG. 3, during thepressurization stage, a gas mixture is pressurized by product gas beingcompressed downward into adsorbent bed 12 by downward movement of secondpiston 52 and second valve 20 being open. Meanwhile first valve 18 isclosed and upward movement of first piston 42 compresses the gas mixtureand increases pressure of the gas mixture on first valve 18.

[0034] During the gas (volume) shift stage, when the gas mixturepressure equals the pressure in adsorbent bed 12, first valve 18 isopened. Because the pressure is the same on both sides of first valve18, no energy is expended in the gas flow past first valve 18. Afterfirst valve 18 is opened, the shift of the gas mixture volume occursupward through adsorbent bed 12. The product gas moves upward, pushingsecond piston 52 upward and thus recovering much of the energy stored bythe previous movement of second piston 52 downward. At the same time asthe gas mixture moves upward, the gas which is selectively adsorbed willbe removed from the gas mixture leaving product gas which adds to theproduct gas already injected in the previous pressurization stage.

[0035] There is a critical step in the timing of the stages just beforethe gas mixture saturates adsorbent bed 12. At this point, second valve20 is closed and first valve 18 remains open. If this is not done, partof the gas mixture will enter with the product gas into second chamber50, reducing the concentration of product gas. During the gas shiftstage, second piston 52 has been retracting, aiding in the gas shift andwithdrawing product gas. The closure of second valve 20 does not resultin an energy loss, as the pressure differential across it is zero. Aftersecond valve 20 closes, some product gas can be withdrawn through secondoutlet 28 for use or storage.

[0036] During the depressurization stage, adsorbent bed 12 isregenerated. First piston 42 retracts and the pressure in adsorbent bed12 falls. The remaining waste gas (gas mixture minus product gas) fromthe gas mixture is exhausted out of first outlet 26. The pressure swingcycle then begins again with a new pressurization stage.

[0037]FIG. 4 is a graph demonstrating the relative timing of gasseparator 10. In particular, the graph shows piston displacement, valveopening and closing, and pressure relative to each piston during thepressure swing cycle. This graph shows displacement of pistons 42 and 52(axis 76) versus the rotation of crankshafts 46 and 56 (axis 78), ininches and degrees respectively. FIG. 4 is similar to FIG. 2, with theaddition of showing the gas pressure found in pumping chambers 40 and50. Lines 80 and 82 show the displacement of first and second pistons 42and 52 respectively. Lines 84 and 86 show the closure and opening offirst and second valves 18 and 20 respectively. Lines 88 and 90 show thepressure within first and second pumping chambers 40 and 50,respectively, during the pressure swing cycle.

[0038] Engineering refinements can be added to the piston motion, suchas, a cam drive of the pistons and valves to provide slight pauses inthe pistons during cycling, with a pause time to a particular valveshape and opening speed. These refinements further improve theefficiency, as would be recognized by one skilled in the art. Thus,changes can be made to the configuration of this invention, but theproper parameters for the gas separation process must take into accountthe critical timing interaction described.

[0039] The pressurization stage begins at a zero degree position ofcrank shafts 46 and 56 with second valve 20 open. Product gas acts topressurize adsorbent bed 12 as second piston 52 moves downward in acompression stroke. The peak pressure is reached at approximately 140degrees. Simultaneously, first piston 42 is rising upward in acompression stroke pressurizing the gas in first pumping chamber 40while first valve 18 is closed. When the pressure is equal on bothsides, first valve 18 opens. First piston 42 continues to its peakpressure at approximately 175 degrees, lagging second piston 52 by about35 degrees. At this point, while both valves 18 and 20 are open, the gasmixture is shifted upward in adsorbent bed 12. Note that second piston52 aids in the gas shift from about 110 degrees to about 180 degreeswhile it is moving in the same direction as first piston 42.

[0040] At approximately 190 degrees, second valve 20 closes andconcentrated product gas is removed from adsorbent bed 12. At about 200degrees in the cycle, first piston 42 is changing direction and thedepressurization stage has begun. At approximately 340 degrees, firstvalve 18 closes and second valve 20 opens, flushing the gas mixturereleased from adsorbent bed 12. A new gas mixture is brought into theadsorbent bed 12 as first piston 42 rises, first valve 18 is closed andsecond valve 20 opens, beginning the pressure swing cycle again.

[0041]FIG. 5 is a diagram of gas mixture 64 and product gas 66 inadsorbent bed 12 over time. FIG. 5 is shown along the same time cycle asFIG. 4. Product gas 66 is indicated by the dense crosshatched area inthe diagram. Gas mixture 64 is indicated by the lightly crosshatchedarea in the diagram, and may also contain the gas mixture without theproduct gas and possibly some diluted product gas. The diagram shows theshallow area of product gas 66 moving in and out of the upper layers ofadsorbent bed 12. The largest volume of product gas 66 in the bed ataround 110 degrees, when the pressure on the gas from second piston 52is at its greatest. From approximately 200 degrees to approximately 360degrees product gas 66 is extracted and has moved to second pumpingchamber 50, as shown by the lack of dark crosshatching during thisperiod. At this time adsorbent bed 12 is depressurized and waste gas ismoved into first pumping chamber 40.

[0042] The type of gas mixture and the adsorbent bed used must be takeninto account as a variable when determining the design parameters.Another factor that has not been mentioned is the effect of the virtualvolume created by the adsorbent bed. For example, when an adsorbent bedis selective for nitrogen this creates a volume effect that is threetimes the actual volume in the adsorbent bed for the selected adsorbedgas. The effect is slightly less than 2 for the product gas, which inthis case would be oxygen. Also the effects of the reduced volume forthe product gas after removal from the gas mixture must be accounted forin the design. The sizing of the piston displacement for the largerfirst piston 42 compensates for the former factors.

[0043] The present invention maximizes the ratio of product gas volumeto energy loss, while minimizing the ratio of mechanical volume of theseparator to the product gas volume. In particular, the desired ratioswere determined by utilization of an adaptive computer program thatmanipulated five variables of a simulated machine as it operated.

[0044] The following steps were taken to minimize energy loss and tomaximize product gas volume:

[0045] (1) Gas flow resistance was decreased by varying the width tolength ratio of the adsorbent bed, as resistance parallel to flow variesas the cube of length.

[0046] (2) Energy losses were decreased in thepressurization/depressurization process by converting the potentialenergy of the compressed gas into usable kinetic energy to move thepistons.

[0047] (3) Valve losses of energy were decreased by opening the valvesonly when the pressure on opposite sides of the valves was equalized.

[0048] (4) Losses by temperature changes due to adsorption anddesorption were decreased by reducing the size of the adsorption bed andby running rapid cycles (typically in the range of about 10 cycles persecond) such that the average thermal change is approximately zero.

[0049] (5) The piston stroke volume was determined based upon the typeof gas mixture and adsorbent bed used as well as the product gas volumeproduced.

[0050] Although the present invention has been described with referenceto preferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope or the invention.

1. A method for operating an oxygen concentrator, the oxygenconcentrator including an adsorbent bed having a separation chamber withfirst and second ports, a first pumping chamber connected to the firstport with a first valve, a first piston in the first pumping chamber, asecond pumping chamber connected to the second port with a second valve,and a second piston in the second pumping chamber, the methodcomprising: closing the first valve; opening the second valve; movingthe first piston toward the adsorbent bed; moving the second pistontoward the adsorbent bed; opening the first valve; moving the secondpiston away from the adsorbent bed; closing the second valve; and movingthe first piston away from the adsorbent bed.
 2. The method of claim 1wherein operation of the oxygen concentrator separates oxygen fromambient air.
 3. The method of claim 2 wherein ambient air is introducedinto the adsorbent bed from the first port and oxygen exits theadsorbent bed from the second port.
 4. The method of claim 1, andfurther comprising: compressing ambient air within the first pumpingchamber with the first piston; pumping oxygen into the adsorbent bedfrom the second pumping station with the second piston; introducingambient air into the adsorbent bed from the first pumping chamber; andwithdrawing oxygen from the adsorbent bed into the second pumpingchamber.
 5. A method for separating oxygen from ambient air using anadsorbent bed having a feed end and a product end, the methodcomprising: compressing ambient air at a feed end of the adsorbent bedwherein a first pressure is created outside of the adsorbent bed;compressing oxygen into the adsorbent bed from the product end of theadsorbent bed wherein a second pressure is created within the adsorbentbed; equalizing the first pressure and the second pressure such that theambient air moves from the feed end into the adsorbent bed; lowering thepressure within the adsorbent bed wherein the oxygen separates from theambient air; compressing ambient air into the adsorbent bed from thefeed end to push the oxygen from the adsorbent bed through the productend into a product chamber connected to the adsorbent bed at the productend; depressurizing the adsorbent bed; and withdrawing the oxygen fromthe product chamber.
 6. The method of claim 5 wherein a feed chamber isconnected at the feed end with a first piston disposed in the feedchamber and a first valve separating the feed chamber from the adsorbentbed to regulate the flow of ambient air with respect to the adsorbentbed, and a second piston is disposed in the product chamber with asecond valve separating the product chamber from the adsorbent bed toregulate the flow of oxygen with respect to the adsorbent bed.
 7. Themethod of claim 6 wherein the compressing ambient air step furthercomprises moving the first piston toward the adsorbent bed wherein thefirst valve is closed to create the first pressure outside the adsorbentbed.
 8. The method of claim 6 wherein the compressing oxygen into theadsorbent bed step further comprises moving the second piston toward theadsorbent bed wherein the second valve is open to create the secondpressure within the adsorbent bed.
 9. The method of claim 6 wherein whenthe first and second pressures are equalized, the first valve opensthereby allowing ambient air to enter the adsorbent bed.
 10. The methodof claim 6 wherein the lowering the pressure within the adsorbent bedstep further comprises moving the second piston away from the adsorbentbed.
 11. The method of claim 6 wherein the compressing ambient air intothe adsorbent bed step further comprises moving the first piston towardthe adsorbent bed wherein the first valve is open and the second valveis open.
 12. The method of claim 6 wherein the depressurizing stepfurther comprises moving the first and second pistons away from theadsorbent bed.
 13. The method of claim 6 wherein the withdrawing oxygenstep further comprises moving the second piston away from the adsorbentbed wherein the second valve is closed.
 14. An oxygen concentratorcomprising: an adsorbent bed including a separation chamber with firstand second ports and a molecular sieve material contained in theseparation chamber; means for regulating the flow of ambient air withrespect to the adsorbent bed; means for regulating the flow of oxygenwith respect to the adsorbent bed; and means for cycling the oxygenconcentrator through a pressure swing cycle such that oxygen isseparated from ambient air within the adsorbent bed.
 15. The oxygenconcentrator of claim 14 wherein the means for regulating the flow ofambient air comprises a first pumping station connected to the firstport.
 16. The oxygen concentrator of claim 15, and further comprising: afirst piston disposed in the first pumping station; and a first valvelocated between the first pumping station and the adsorbent bed.
 17. Theoxygen concentrator of claim 14 wherein the means for regulating theflow of oxygen comprises a second pumping station connected to thesecond port.
 18. The oxygen concentrator of claim 17, and furthercomprising: a second piston disposed in the second pumping station; anda second valve located between the second pumping station and theadsorbent bed.
 19. The oxygen concentrator of claim 14 wherein the meansfor cycling the oxygen concentrator through a pressure swing cycleincludes a drive system that coordinates operation of the means forregulating flow of ambient air and the means for regulating flow ofoxygen.
 20. A portable oxygen concentrator for separating oxygen fromambient air, the oxygen concentrator comprising: an adsorbent bedincluding a separation chamber with feed end and a product end and amolecular sieve material contained in the separation chamber; a firstpumping station connected to the feed end to regulate flow of ambientair with respect to the adsorbent bed; a second pumping stationconnected to the product end to regulate flow of oxygen with respect tothe adsorbent bed; a drive system to coordinate operation of the firstpumping station and the second pumping station in a pressure swingcycle; and a battery operated power source.
 21. The oxygen concentratorof claim 20 wherein the drive system operates the pressure swing cyclesuch that average thermal change is approximately zero.
 22. The oxygenconcentrator of claim 21 wherein the drive system runs about 10 cyclesper second.